To tie a shoelace into a knot is a relatively simple affair. Tying a
knot in a field is a different story, because the whole of space must be
filled in a way that matches the knot being tied at the core. The
possibility of such localized knottedness in a space-filling field has
fascinated physicists and mathematicians ever since Kelvin's 'vortex
atom' hypothesis, in which the atoms of the periodic table were
hypothesized to correspond to closed vortex loops of different knot
types. An intriguing physical manifestation of the interplay between
knots and fields is the possibility of having knotted dynamical
excitations. I will discuss some remarkably intricate and stable
topological structures that can exist in light fields whose evolution is
governed entirely by the geometric structure of the field. A special
solution based on a structure known as a Robinson Congruence that was
re-discovered in different contexts will serve as a basis for the
discussion. I will then turn to hydrodynamics and discuss topologically
non-trivial vortex configurations in fluids.